Avalanche forecast

Snoqualmie Pass

Northwest Avalanche Center
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Avalanche danger

Sun
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

6:00am Forecast Update: Upgraded the Below Treeline Band to Considerable due to increased confidence in heavy precipitation.

Snoqualmie Pass could experience rain, freezing rain, strong winds, and heavy snow all within a short timeframe, making Saturday a dangerous day to be in the mountains. Rapid changes in the weather often lead to avalanches. Constantly re-evaluate your plan for the day as conditions change and when in doubt, stay off slopes 35 degrees and steeper.

 

 

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Heavy snowfall rates, warming temperatures, and strong to extreme winds will all aid in the development of Storm Slabs on Saturday at Snoqualmie Pass. Slabs will be thicker above the rain/snow line and in wind-prone areas. We are expecting an upside-down snowpack to develop throughout the zone by day’s end, which is a recipe for slab avalanches. Use small test slopes and hand pits as you travel to determine the depth and reactivity of the new storm snow. If you find heavy snow overlaying light snow or notice signs of wind-effect such as textured surfaces, blowing snow, or freshly formed cornices, stay off slopes over 35 degrees.

    (12/10/2021)

    Simple snowpack tests like this can help you to identify interstorm layers that may be of concern. This layer was identified about 10cm down where warming had caused an "upside-down" snowpack to start to develop on Friday, 12/10.

    Photo: Robin Pendery

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Expect wet avalanche activity to occur below the rain/snow line on Saturday at Snoqualmie Pass. The timing and elevations for these avalanches are hard to predict, but your easiest indicator will be the type of precipitation falling from the sky. If you encounter rain falling on cold dry snow, you should anticipate wet avalanche activity. These avalanches often start at a point and are capable of entraining snow, triggering slabs, and traveling far distances as they descend. Rollerballs, pinwheels, and fan-shaped avalanche debris are visual clues you can use to help identify these avalanches. If you notice signs of wet avalanches or find unconsolidated wet snow, stay off of terrain features greater than 35 degrees and avoid traveling underneath steep rocky slopes and cliff bands where snow could shed from above.

    (12/19/2020) Example: Look for slides starting at a point, fan-shaped debris, rollerballs, and pinwheels to help you identify these avalanches.

     

    Photo: Andy Harrington

Avalanche Discussion

It’s going to be a wet and wild day at Snoqualmie pass on Saturday. Rain, freezing rain, snow, and strong to extreme winds are all on the table throughout the forecast period. With so many possible weather inputs and low confidence in the way they are going to play out, this forecast is not without uncertainty. 

What we can say with certainty is that Saturday will be a dangerous day to be in the mountains.  The expected rising freezing levels should be the key to driving instability at all elevation bands. At times, precipitation rates will be heavy and the winds are expected to increase throughout the day. These factors will lay down heavy thick snow or create Wind Slabs on top of our existing low-density snowpack, creating an upside-down scenario that is a perfect recipe for avalanches. Don’t be surprised to see some natural avalanche activity as the snowpack adjusts to this rapid loading. We are most concerned about the new/old snow interface on Saturday, but multiple reports and recent avalanches around the zone indicate that a layer of graupel/rimed precipitation particles about 40cm down is a potential point of failure to keep on your radar.  The uncertainty around the weather means that you will need to constantly be re-evaluating your plan for the day. Be sure to make constant observations and do your due diligence before stepping into avalanche terrain. If the snow is piling up more than expected or during periods of high precipitation rates, you could find that the hazard approaches High at upper elevations. 

At lower elevations, the threat of rain and freezing rain will drive a different type of hazardous day. Although the snowpack is still building, there is enough snow at mid to lower elevations for avalanches to occur. Even in popular hiking areas along the I-90 corridor, expect rain to fall on snow that is deep enough for avalanche activity in recent days. Any rain will only help to further highlight our early season snowpack where open creeks, shallowly buried objects, and other natural hazards exist.

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